As increasing amount of information is required by information apparatuses, audio-visual apparatuses, and the like in recent years, information recording media such as optical disks have attracted attention for the advantages thereof such as the ease of access to data, large data storage capacity and possibility of reducing the size of apparatuses, and the recording density thereof has become increasingly higher. As the means for increasing the recording density of an optical disk, for example, such a recording and reproduction head has been put into use that uses laser beam having wavelength of about 400 nm and a condenser lens having numerical aperture (NA) of 0.85 for focusing the laser beam. Use of such a recording and reproduction head enables it to achieve a storage capacity of about 25 GB with a single-layer information recording medium (a medium having one recording layer). A storage capacity of about 50 GB can be achieved with an optical recording medium having two recording layers. For the purpose of increasing the storage capacity even further, multi-layer information recording media having three or more recording layers have been proposed.
FIG. 2 is a sectional view showing a part of a multi-layer information recording medium of the prior art. This drawing shows a section along a diameter of the disk-shaped recording medium, where the left-hand end of a substrate 301 corresponds to inner circumference and the right-hand end of a substrate 301 corresponds to outer circumference. The multi-layer information recording medium is constituted from a first substrate 201 (this substrate may also be referred to as signal substrate) having pits and/or guide grooves formed in configuration of recesses and bumps by transferring process on one side thereof, a first thin film layer 202 disposed on the surface of the first substrate 201 whereon the signals are formed, a resin layer 203 formed on the first thin film layer 202, a second thin film layer 204 formed on the resin layer 203, and a transparent substrate 206 laminated on the second thin film layer 204 by means of a transparent layer 205. The resin layer 203 has pits and/or guide grooves formed in configuration of recesses and bumps by transferring process on the side thereof that is opposite to that bonded with the first thin film layer 202 (namely on the surface where the second thin film layer 204 is formed). In this specification, a plurality of layers having the same function will be identified as “first,” “second,” and so on by counting from one that is nearest to the substrate 201.
The resin layer 203 optically separates the first thin film layer 202 from the second thin film layer 204 so as to ensure that, while information is recorded on or reproduced from one of these thin film layers, information will not be recorded on or reproduced from the other thin film layer. The pits and/or guide grooves formed on the resin layer 203 serve to guide the recording and/or reproducing light beam when information is recorded on the second thin film layer 204 or information is reproduced from the second thin film layer 204. As such, the resin layer 203 may also be referred to as an intermediate layer or a second signal substrate.
The first substrate is formed by injection compression molding process or the like by using a stamper. The first substrate 201 has pits and/or guide grooves on one side thereof which are formed by transferring a configuration of recesses and bumps formed on the stamper surface. The pits and/or guide grooves provide basic information during recording and reproduction of the signals, and the surface having them formed thereon may be called the information surface. The substrate having the pits and/or guide grooves formed thereon may be referred to as the “signal substrate.” A thin film layer having such properties that enable it to optically record and reproduce information, including a recording film and a reflective film, is formed on the information surface, thereby forming an information recording layer. The first substrate 201 has a thickness of about 1.1 mm.
The first thin film layer 202 and the second thin film layer 204 include the recording film and the reflective film, as described above, and are formed by a method such as sputtering or vapor deposition. The second substrate (resin layer) 203 is formed from a photo-setting resin by a spin coating method or a screen printing process. The pits and/or guide grooves are formed by laminating a transfer substrate, that has a configuration of recesses and bumps corresponding to the pits and/or guide grooves to be formed, with the side having the configuration of recesses and bumps in contact with the photo-setting resin, before the photo-setting resin is cured. The process of forming the pits and/or guide grooves is completed by peeling the transfer substrate from the photo-setting resin after curing the photo-setting resin by exposing it to light.
Recording of information on the optical recording medium illustrated and reproduction of information from the optical recording medium are carried out by directing recording or reproduction laser beam through the transparent substrate 206. Thus the transparent substrate 206 is formed from a material that is transparent (transmissible) to the recording or reproduction light. The transparent substrate 206 has thickness of about 0.1 mm so that a recording and reproduction head whose numerical aperture is large can be used. The transparent layer 205, provided to bond two substrates 206 and 207 to each other, is formed from a photo-setting resin or an adhesive such as a pressure-sensitive adhesive. The transparent substrate 206 and the transparent layer 205 may be collectively called a cover layer. The cover layer may also be constituted from the transparent resin layer only, without laminating the transparent substrate.
In the multi-layer information recording medium, as described above, the resin layer and the cover layer are generally formed by a spin coating method by using an ultraviolet curable resin or the like (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 2005-259331).
Japanese Unexamined Patent Publication (Kokai) No. 2002-63736 discloses an optical information medium that has two or more information holding layers and at least two ring-shaped resin layers that have a ring-shaped ridge on the inner circumference, wherein the resin layers are placed one on another stepwise in such a way that each resin layer does not cover the ring-shaped ridge of another layer. When the resin layers are placed stepwise by the spin coating method for forming the resin layers successively, the resin is not prevented by the ring-shaped ridge from spreading, and thickness of the resin layer does not deviate from the design value in the vicinity of the inner circumference of the disk.